Photocell bias circuit

ABSTRACT

Receiver circuits are provided for detecting a target light source. The circuits include a photocell arranged to detect light impinging thereupon, and a biasing circuit that maintains a constant reverse voltage across, the photocell irrespective of the photocell output. This maintains the capacitance of the photocell at a substantially constant value, and results in faster response times from the photocell. Further, the present invention provides circuits for effectively removing ambient conditions such as daylight from the output. Also, the present invention reduces the amount of signal loss between the photocell output and an amplifier due to loading at the input stage of the amplifier.

BACKGROUND OF THE INVENTION

The present invention relates in general to a light detecting andamplifying circuit and in particular to a light detecting circuit thatprovides solar cancellation.

Photocells are devices that produce an output current generallyproportional to the irradiance of the light impinging thereon. As such,photocells provide a convenient means to measure the intensity ofelectromagnetic radiation in the visible and infrared regions. However,the output signal generated by the photocell is typically weak; thussome sort of amplification and/or signal conditioning is used beforefurther processing of the output current by additional signal processingcircuitry.

Photocells generally comprise an intrinsic depletion layer sandwichedbetween an n-type doped region (the cathode) and a p-type doped region(the anode). If the photocell is reverse biased (a positive voltageapplied from the cathode to the anode), the depletion area increases andthe capacitive effects of the photocell decrease. A typical reverse biaslight receiving circuit 10 is illustrated in FIG. 1. Light impinging onthe photocell 12 causes a current (I_(PC)) 14 to flow from the anode ofthe photocell 12 into the load resistor (RL) 16. The current 14 istypically weak and varies depending upon the intensity of light on thephotocell. The load resistor 16 is provided to convert the current 14into a voltage (V_(PC)) 20. Further, amplifier 22 is provided to amplifythe voltage across the load resistor 16.

Photocells typically exhibit a parasitic capacitance that can beexpressed by the well-known formula: $C = \frac{ɛ_{s}A}{x_{T}}$

Where ε_(s) is the permitivity of substrate, A is the junction area, andX_(T) is the width of the depletion region.

It is desirable to minimize this capacitance to improve the speed andresponse time of the photocell. It should be clear from the aboveformula that increasing the width of the depletion region of thephotocell reduces the parasitic capacitance. This is accomplished byapplying a reverse voltage between the cathode and anode. As such, abias voltage (V_(Bias)) 18 is applied to the cathode of the photocell.

As a practical matter, the selection of a value for the load resistor 16can prove a limiting factor in the circuit performance. For example, theload resistor 16 forms a parallel circuit with the input impedance ofthe amplifier 22. An appreciable amount of the output of the photocell12 can thus be lost across the load resistor 16 due to loading effectsif the resistance value of the load resistor 16 is chosen too small. Theamount of signal loss due to the loading effects of the load resistor 16is the percentage of the load resistor 16 in parallel with thetransimpedance gain of the amplifier 22. On the other hand, higherresistance values result in higher resistor thermal noise, which cancompromise the accuracy of the output by the photocell 12, especiallywhen the output current is weak. Also, the higher the value of the loadresistor 16, the more limited the dynamic range of the photocell 12.This is seen by the observation that should the voltage 20 across theload resistor 16 (computed from Ohm's law as the photocell outputcurrent 14 times the resistance value of the load resistor 16), exceedthe bias voltage 18, the photocell 12 will no longer be reverse biased,and the capacitance of the photocell increases. To prevent this fromhappening, the bias voltage 18 is known to exceed 50 volts in someapplications. Such a solution is inefficient, especially when designingthe circuit for battery powered portable devices.

Another disadvantage of the prior art circuit of FIG. 1 lies in theobservation that the reverse voltage between the cathode and anode ofthe photocell, and thus the parasitic capacitance of the photocell,changes as the intensity of light impinging on the photocell changes.The reverse voltage that biases the photocell is the bias voltage 18minus voltage 20 across the load resistor 16. As pointed out above, aslight intensity impinging on the photocell 12 increases, the photocelloutput current 14 increases, and thus the voltage 20 increases. Thislowers the effective reverse voltage, and thus increases the capacitanceof the photocell. As a result, response time of the photocell can becomesluggish and the photocell may become ineffective at capturing shortduration pulses of light.

Yet another disadvantage of the circuit of FIG. 1 is that any noise inthe power supply that provides the bias voltage 18 is seen as an outputof the photocell 12 and amplified by the amplifier 22.

Accordingly, there is a need for a circuit that provides a constantreverse voltage to a photocell. Further, there is a need for a circuitthat can maintain the constant reverse voltage by using a low voltagesupply.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of previously knownreceiver circuits by providing a circuit that maintains a constantreverse voltage across one or more photocells. This maintains thecapacitance of the photocell at a substantially constant value, and fastphotocell response times are realized. Further, the present circuitsreduce the amount of signal loss due to loading at the input stage ofthe amplifier, and effectively remove noise, including ambientconditions such as daylight.

In accordance with one embodiment of the present invention, a singleoperational amplifier is used to maintain a photocell at a constantreverse voltage. The photocell is connected to the circuit such that theanode is coupled to ground, and the cathode is tied to the invertinginput of an operational amplifier (op-amp). The non-inverting input ofthe op-amp is tied to a reference voltage, which is adjusted to thedesired bias voltage. Further, an inductor is provided in a negativefeedback loop between the inverting input and output of the op-amp. Thephotocell is also coupled to a transimpedance amplifier through acapacitor. The inductor is seen by the photocell as a low impedance loadfor low frequency signals. Thus, the effects of daylight, which areobserved by the photocell as d.c. or low frequency signals are,effectively buffered. Further, the photocell sees the inductor as a highimpedance load at high frequencies. Thus a substantial portion of thesignal of interest from the photocell is delivered to the transimpedanceamplifier.

According to another embodiment of the present invention, a photocell isconnected to the circuit such that the anode is coupled to ground, andthe cathode is tied to the inverting input of an operational amplifier(op-amp). The non-inverting input of the op-amp is tied to a referencevoltage, which is adjusted to the desired bias voltage. Further, aparallel combination of an inductor and a capacitor is provided in anegative feedback loop between the inverting input and output of theop-amp. The photocell is also coupled to a transimpedance amplifierthrough a capacitor.

The parallel combination of the inductor and capacitor are tuned suchthat the feedback loop of the op-amp provides a high impedance load atfrequencies of interest. Thus a substantial portion of the signal fromthe photocell is delivered to the transimpedance amplifier. Further, theparallel combination of the inductor and capacitor provide a lowimpedance load at frequencies of no interest. The inductor is seen bythe photocell as a low impedance load for low frequency signals. Thusthe effects of daylight, which are observed by the photocell as d.c., orlow frequency signals are effectively buffered by the inductor. Thecapacitor placed in parallel with the inductor is seen by the photocellas a low impedance load for very high frequency signals. Thus highfrequency noise is effectively buffered and reduced from appearing atthe output of the transimpedance amplifier.

It is thus an object of the present invention to provide a circuit thatsupplies a photocell constant reverse voltage that remains substantiallyconstant, irrespective of the photocell output current.

It is an object of the present invention to provide a circuit thatactively filters the effects of daylight from the receiver.

It is an object of the present invention to provide a circuit thatbuffers the output current from the photocells when the output comprisesfrequencies of no interest.

It is an object of the present invention to provide a circuit having avery high impedance buffer at the frequencies of interest such thatthere is minimal signal loss in the transfer of the signal of interestfrom the photocell to the amplifier.

Other objects of the present invention will be apparent in light of thedescription of the invention embodied herein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of the preferred embodiments of thepresent invention can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals, and in which:

FIG. 1 is a schematic illustration of a receiver circuit according tothe prior art;

FIG. 2 is a schematic illustration of a receiver circuit according toone embodiment of the present invention;

FIG. 3 is a schematic illustration of a receiver circuit according toanother embodiment of the present invention;

FIG. 4 is a block diagram of a receiver circuit according to oneembodiment of the present invention; and,

FIG. 5 is a block diagram of a receiver circuit according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration, and not by way oflimitation, specific preferred embodiments in which the invention may bepracticed. It is to be understood that other embodiments may be utilizedand that logical, mechanical and electrical changes may be made withoutdeparting from the spirit and scope of the present invention.

Referring to FIG. 2, an improved circuit 100 for a laser receiver isillustrated. A light detector 102, such as a photocell, comprises aphotodiode having an anode 104 coupled to ground potential, and acathode 106 coupled to a bias voltage (+V_(Bias)) 108. The lightdetector 102 outputs a current I_(PC) based upon the intensity of lightmeasured thereby.

A first operational amplifier (op-amp) 110 comprises a non-invertinginput 112 coupled to a first reference voltage (V₁+) 114, and aninverting input 116 coupled to output 118 by a negative feedback loop120. The negative feedback loop 120 includes an inductor 122. Theinverting input 116 is also coupled to the cathode 106 of the lightdetector 102.

A second op-amp 126 comprises a non-inverting input 128 coupled to asecond reference voltage (V₂+) 130, and an inverting input 132 that iscoupled to output 134 by a negative feedback loop 136. The negativefeedback loop 120 includes a resistor 138. The inverting input 132 isalso coupled to the cathode 106 of the light detector 102 throughcapacitor 140. The output signal or voltage output (V_(out)) 142 of thecircuit 100 is measured across the output 134 of the second op-amp 126.

The value of the second reference voltage (V₂+) 130 will depend upon thetype of op-amps used. For example, if the second op-amp 126 includesboth positive and negative supply voltages, the second reference voltage(V₂+) 130 may be tied to ground potential. Where the power supplied tothe second op-amp 126 comprises a single positive voltage, then thesecond reference voltage (V₂+) 130 is selected somewhere between groundpotential and the supply voltage. For example, the second op-amp 126 istypically powered by a +5 volt supply. Under such an arrangement, thesecond reference voltage (V₂+) 130 is selected somewhere between +5volts and ground potential, such as 2.5 volts. It may also be convenientto set the second reference voltage (V₂+) 130 equal to the firstreference voltage (V₁+) 14. For example, where the first referencevoltage (V₁+) 114 is 2 volts, setting the second reference voltage (V₂+)130 to 2 volts simplifies circuit design. Further, because the signal isgoing to have a mostly positive output, increased positive headroom isrealized.

The operation of the circuit 100 will now be described for an exemplaryapplication in the field of surveying. In a typical surveyingapplication, a laser transmitter or other illuminated target produces abeam of coherent light that repeatedly sweeps across a target area. Thecircuit 100 is positioned within the target area, and the light detector102 is used to sense the presence of the light. One or more photocellsare well suited for use as the light detector 102 in this applicationbecause a typical photocell outputs a current that is proportional to,or at least related to the intensity of light that impinges thereupon,and generally has low internal noise and intrinsically fast responsetimes thus allowing the measurement of very short light pulses.Essentially, the photocell detects the amplitude variations of lightimpinging thereon, and must be capable of detecting spectral frequencieswithin a predetermined frequency range that corresponds to the beam ofcoherent light that sweeps across the target area.

The portion of the circuit 100 defined by the first op-amp 110 providestwo functions. The first reference voltage (V₁+) 114 in combination withthe first op-amp 110 provides a constant bias circuit arranged tomaintain the light detector 102 in a reverse biased state such that areverse voltage (a voltage having a positive polarity when measured fromthe cathode 106 to the anode 104) remains substantially constantirrespective of the output signal generated by the light detector 102.Also, the first op-amp 110 and negative feedback loop 120 define asignal filter or first variable impedance load that serves as a bufferfor the output current of the light detector 102. The signal filterperforms the task of filtering the measured light having spectralfrequencies detected outside the predetermined frequency range such thatnoise and ambient light conditions are substantially attenuated, orignored.

The constant bias circuit takes advantage of the principle of thevirtual short circuit for ideal op-amps. That is, the voltage betweenthe non-inverting input 112 and the inverting input 116 is ideally zerovolts. This means that the first reference voltage (V₁+) 114 applied tothe non-inverting input 112 will appear at the inverting input 116.Because the inverting input 116 is tied to the cathode 106 of the lightdetector 102, the first reference voltage (V₁+) 114 provides the biasvoltage to the light detector 102. That is, under steady stateconditions, the bias voltage (+V_(Bias)) 108 is substantially equal tothe first reference voltage (V₁+) 114. Because the anode 104 of thelight detector 102 is tied to ground, the bias voltage (+V_(Bias)) 108maintains a constant reverse bias across the light detector. As thelight detector 102 begins to output current in response to detectedlight, the inverting input 116 serves to maintain and drive the biasvoltage (+V_(Bias)) 108 to maintain a substantially constant valuedetermined by the first reference voltage (V₁+) 114.

The constant reverse bias across the light detector 102 allows the sizeof the depletion region of the light detector 102, and thus theparasitic capacitance of the light detector 102, to be maintained atsubstantially constant and relatively low values. The capacitance of thelight detector 102 affects the response time of the light detector 102,and hence the response time of circuit 100. By maintaining thecapacitance of the light detector 102 at a substantially constant value,the response time of the circuit will remain predictable irrespective ofthe intensity of the light impinging upon the light detector 102.Further, the parasitic capacitance of the light detector 102, and thusthe dynamic range of the light detector 102, can be tuned to obtainspecific performance characteristics by varying the first referencevoltage (V₁+) 114.

The present invention can achieve suitable performance with the biasvoltage (+V_(Bias)) 108 set to relatively low voltage levels. Thevoltage level may even be within the power supply rails of the op-amp110. For example, the first reference voltage (V₁+) 114 can be set to avoltage less than 5 volts, and in one embodiment, the first referencevoltage (V₁+) 114 is set to a value around 2 volts. Thus it is practicalto use circuit 100 in battery powered circuits that have significantlyimproved operational times between battery changeovers. It will beobserved that the present invention does not rely on a load resistor inseries with the anode of the light detector to measure the outputcurrent of the light detector, as required by the prior art. Thus, withthe circuit of the present invention, the large voltages required by theprior art to maintain the light detector in a reverse biased state areno longer necessary.

The first reference voltage(V₁+) 114 is preferably selected as a voltagewithin the power supply rails of the op-amp 110. Accordingly, the powersupply rejection ratio, or power supply noise rejection of the firstop-amp 110, serves to condition and buffer the first reference voltage(V₁+) 114 appearing at the inverting input 116. This conditioningprovides a reference voltage (the bias voltage (+V_(Bias)) 108) that isstable and less susceptible to noise, ripple, and other adverse effects.Thus, the effects of the noise are minimized before the noise reachesthe second op-amp 126.

In operation, first assume that no light is incident on the lightdetector 102. Under this condition, the light detector 102 does notoutput any current, and the bias voltage (+V_(Bias)) 108 is maintainedat a constant direct current (d.c.) voltage substantially equal to thefirst reference voltage (V₁+) 114. At d.c., the capacitor 140 issaturated and thus modeled by an open circuit. Thus, the second op-amp126 is effectively isolated from the light detector 102.

In typical use, however, the light detector 102 will be exposed todaylight. The circuit 100 must therefore discriminate between thecurrent output by the light detector 102 representing the intensity ofthe daylight, and the current produced by the intensity of the laserlight that impinges upon the light detector 102. In other words, theambient daylight represents a signal of no interest, and the laser lightrepresents a signal of interest.

Ambient daylight impinging upon the light detector causes the lightdetector to produce a fairly steady d.c. current output, or at least avery low frequency output. For example, on a clear day, the sun shinesall the time, thus the light impinging on the light detector 102 willremain substantially constant over the time that the device isoperating. As such, the sunlight will “look” generally like a d.c.current output by the light detector 102. On a cloudy day, or in lessthan perfect environmental conditions, the intensity of the ambientlight may change, but the change happens gradually over the course oflong periods of time relative to the time upon which the laser impingesthe light detector 102. Thus the effects of ambient daylight under theseconditions will still “look” like d.c. current, or low frequencycurrent, for example on the order of a few hertz or less.

The swept laser impinges on the light detector 102 periodically duringshort Intervals of time. Thus the light detector 102 will output acurrent signal representing the laser and having relatively highspectral frequencies in comparison to the background light. For example,the laser component of the output of the light detector 102 may have afrequency that is on the order of 100 kilohertz, and even into themegahertz range. As such, the output of the light detector 102 willgenerally have two components. A slowly varying, or d.c. componentrepresenting the ambient light, and an alternating current (a.c.)component representing the laser light (which is the component ofinterest).

When the circuit 100 is exposed to ambient light, the light detector 102will output a current signal representing the intensity of the measuredlight. As explained above, this signal approximates a d.c. signal, orlow frequency signal. There are essentially only four paths for thecurrent output by the light detector 102 to travel. The first direction144 is a path to ground, and negligible current will flow in thisdirection.

The second direction 146 is into the inverting input 116 of the firstop-amp 110. Based upon the principle of the virtual short circuit, thecurrent entering either the non-inverting input 112 or the invertinginput 116 of the op-amp 110 is zero, thus negligible current enters theinverting input 116.

The third direction 148 that the current can flow is towards the secondop-amp 126. As pointed out above, the capacitor 140 looks like an opencircuit to d.c. signals. At low frequencies, the capacitive reactance ofthe capacitor 140 is large. Therefore the input impedance looking intothe second op-amp 126 is large. The capacitor 140 and the op-amp 126thus appear as a high impedance load, and negligible current travels inthe third direction.

The fourth direction 150 is essentially a path through the feedback path120 of the first op-amp 110. The op-amp 110 and negative feedback path120 simulate an inductive load for the light cell 102. The inductor 122is modeled as a short circuit for d.c. signals. At low frequencies, theinductive reactance of the inductor 122 is small and the inductorappears as a very small impedance. The output 118 of the first op-ampalso has a low impedance.

The signals from the light detector 102 that represent typical ambientdaylight conditions, or signals of no interest, see an open circuit inthe case of d.c. signals, and a very high impedance in the case of lowfrequency a.c. signals when looking in the third direction 148, and alow impedance looking in the fourth direction 150. As such, the currentoutput by the light detector 102 in response to ambient sunlight issubstantially diverted to, or buffered by, the feedback path 120 of thefirst op-amp 110, and is substantially attenuated. The first op-amp 110is typically capable of handling relatively high current levels. Thusthe first op-amp 110 can satisfactorily accommodate the current outputacross the entire dynamic range of the light detector 102.

As the laser light is swept and momentarily impinges upon the lightdetector, the current output by the light detector 102 representing thelaser light component looks like a relatively high frequency a.c.signal, and represents a signal of interest. Negligible current willtravel in the first and second directions 144, 146 for reasons statedabove. At relatively high frequencies, the capacitive reactance of thecapacitor 140 is small, thus the third direction 148 “sees” a lowimpedance load. Likewise, at relatively high frequencies, the inductivereactance of the inductor 122 is large, thus the fourth direction 150“sees” a high impedance load. As such, a majority of the signal ofinterest will couple to the second op-amp 126. As such, the circuit 100provides a very high impedance buffer (negative feedback loop 120) atthe frequencies of interest such that signal loss in the transfer of thesignal of interest from the light detector 102 to the op-amp 126 isminimized. Thus the output of the light detector 102 is notsubstantially attenuated for signals having a spectral frequency withina predetermined frequency range.

The second op-amp 126 is configured to act as a transimpedanceamplifier. The second op-amp 126 converts the output currentrepresenting the signal of interest into an output voltage, and providesamplification to the signal of interest through resistor 138 in thenegative feedback loop 136. The output of the transimpedance amplifierwill comprise a sequence of voltage pulses corresponding to laser pulsesdetected by the light detector 102. It will be appreciated that thisoutput voltage can then be sent to any one of various configurations ofprocessing circuitry. The present invention provides improvedsensitivity over the prior art use of a load resistor in series with theanode of the light detector because in the present invention, asubstantial portion of the signal output by the light sensor 102 isdelivered to the transimpedance amplifier. In the prior art, a portionof the signal is lost by the percentage of the load resistor in parallelwith the transimpedance gain of the amplifier circuit, as illustrated inFIG. 1.

The output of the transimpedance amplifier is the output current of thelight detector (denoted schematically in FIG. 2 as I_(PC)) times theresistance value of resistor 138 in the negative feedback loop 136 ofthe second op-amp 126. As the feedback resistance of the transimpedanceamplifier is varied, both the gain of the amplifier and the circuitbandwidth changes. As the feedback resistance increases, straycapacitance across resistor 138 and capacitance from the light detector102 increasingly limits the maximum high frequency bandwidth of thecircuit 100. However, at low feedback resistance, the combined effect ofthe phase shift of the second op-amp 126 operating as an invertingamplifier, the phase shift caused by the capacitance of the lightdetector 102, and feedback resistance of resistor 138 cause a peak inthe frequency response of the transimpedance amplifier circuit. As thegain is reduced, the circuit may be driven into oscillation.

Referring to FIG. 3, where like structure is indicated with likereference numerals, an optional capacitor 152 is positioned in parallelwith resistor 138 in the negative feedback loop 136 of the second op-amp126. The capacitor 152 is used to reduce or eliminate the threat ofoscillation in the transimpedance circuit. The capacitor 152 reduces thephase shift at the unity loop gain frequency. The value of capacitor 152is determined from the capacitance of the light detector 102, the inputcapacitance of the second op-amp 126, the open-loop gain/bandwidthproduct of the second op-amp 126, the value of resistor 138, andpreferably from other stray capacitances in the circuit design. Someconsideration must be placed in the selection of the value of thecapacitor 152. If the value of the capacitor 152 is selected too large,the bandwidth of the circuit 100 may be unnecessarily limited. If thecapacitance is too small, the frequency response of the op-amp 126 maycontain an undesirable spike. As an alternative to providing capacitor152, a faster op-amp having a higher frequency first pole in its openloop gain characteristics may be used.

It will be observed that the bias voltage (+V_(Bias)) 108 is maintainedat a constant d.c. voltage, thus the parasitic capacitance seen acrossthe light detector 102 remains constant. The noise gain of the secondop-amp 126 is derived in part, from this parasitic capacitance. Thus,the noise gain structure of the second op-amp 126 remains constantthroughout the operational range of the light detector 102. Referringback to FIG. 1, with prior art circuits such as that described withreference to FIG. 1, the noise gain structure of the amplifier 22 isvariable, and is derived from the parasitic capacitance of the photocell12 divided by the capacitance in the feedback loop of the amplifier 22.At the bias voltage minimum, the parasitic capacitance of the photocell12 is large, thus the noise gain is at a maximum value. Conversely, asthe bias voltage increases, the capacitance of the photocell 12decreases, thus the noise gain structure of the amplifier 22 decreases.

Referring back to FIG. 3, the present invention can maintain a highbandwidth over all frequencies of interest, and thus allows for fasterresponse times over the prior art. As pointed out, the bias voltage(+V_(Bias)) 108 is a constant d.c. voltage, thus the parasiticcapacitance of the light detector 102 remains constant. As such, thereis no delay caused by the need to recharge the parasitic capacitance ofthe light detector 102 in response to light impinging thereon. In priorart circuits such as that described with reference to FIG. 1, thecapacitance of the photocell changes as the bias voltage changes. Theneed to recharge the photocell 12 thus causes limitations in thebandwidth of the circuit.

As shown in FIG. 3, an optional capacitor 154 is placed in parallel withinductor 122 in the negative feedback loop 120 of the first op-amp 110.At very low frequencies, the inductive reactance of the inductor 122 issmall and the inductor 122 appears as a very small impedance. At veryhigh frequencies, the capacitive reactance of the capacitor 154 is smalland the capacitor 154 appears as a very small impedance. The inductor122 and capacitor 154 in parallel form a tuned circuit having acomputable center frequency, and the negative feedback loop 120 of thefirst op-amp 110 appears as a small impedance load both at low and highfrequencies with respect to the tuned center frequency. As such, thenegative feedback loop 120 forms a circuit that can be designed to notchat the target frequency of the laser light such that the negativefeedback loop 120 appears to the light detector 102 as a low impedanceload except for the frequency band of the laser light. At that frequencyband, the negative feedback loop 120 appears to the light detector 102as a high impedance load. The center frequency of the LC circuit and thedetermination of the pole Q factor are easily determined by well-knownformulas.

There are numerous advantages to this configuration. The capacitor 154allows the negative feedback loop 120 to appear as a low impedance loadto very high frequencies, and thus serves to filter high frequency noisein addition to the low frequency and d.c. ambient conditions. The pole-Qdetermines how narrow (selective) the bandpass filter defined by the LCcircuit of inductor 122 and capacitor 154 is. Specifically, as Qincreases, the bandwidth decreases and the filter becomes moreselective. As such, the negative feedback loop 120 can be tuned to looklike a high impedance load to the light detector 102 only for a narrowband of frequencies that corresponds to the expected frequency range ofthe laser light source.

For example, in applications where it is desirable to filter out ambientconditions such as the effects of daylight, the negative feedback loop120 may be configured to provide low pass filtering of DC, and lowfrequency component signals. For example, a lowpass filter cutoffbetween 10 Hertz and 50 Hertz may provide suitable results. However, itwill be appreciated that the application will dictate the cutofffrequency, or the frequencies at which the negative feedback loop 120will appear as a low impedance load. For example, in many surveyingapplications, a cutoff frequency of approximately 10 Kilohertz providessatisfactory results; thus the negative feedback loop 120 is tuned toprovide a lowpass filtering function having a cutoff frequency up to 10Kilohertz. In other words, the negative feedback loop is configured toprovide a low impedance load to signals less than 10 Kilohertz infrequency. If a capacitor is used to also filter high frequencycomponents, it will be appreciated that the upper cutoff frequency willdepend upon the particular application and the bandwidth of the firstop-amp 110. For example, the cutoff frequency may be tuned to 1-2Megahertz for certain surveying applications. However, as pointed outabove, the circuit 100 does exhibit the same bandwidth limitations asthe prior art circuits such as described with reference to FIG. 1.Accordingly, the upper cutoff frequency may be on the order of 12Megahertz or higher.

It is worth noting that the constant bias circuit maintains a constantreverse bias across the cathode 106 and anode 104 of the light detector102 irrespective of light current output. As such, the capacitance ofthe light detector 102 remains substantially constant during operation.However, even if the capacitance of the light detector 102 shouldchange, the capacitance will not affect the impedance characteristics ofthe negative feedback loop 120. This is true whether the simulatedinductor comprises only an inductor 122 in the negative feedback loop120, as shown in FIG. 2, or whether the tuned circuit defined by theparallel combination of inductor 122 and capacitor 154 is used in thenegative feedback loop 120, as shown in FIG. 3. This is because thenegative feedback loop 120 is not connected in parallel with the lightdetector 102.

Referring to FIG. 4, a light detecting system 200 comprises at least onephotocell, and can include a bank or banks of photocells 202. A constantbias circuit 204 couples to the photocells 202 and keeps the photocellsat a substantially constant reverse bias irrespective of the outputcurrents generated by the photocells 202 due to the intensity of lightimpinging thereon. A signal filter circuit 205 is coupled to thephotocells 202. The filter circuit can be any filter arranged to filternoise, ambient conditions such as daylight, and other signals of nointerest. Although discussed above with reference to simple inductiveand capacitive circuits, it will be appreciated that the filtercircuitry can comprise higher order filters. The specific applicationwill direct the frequency response and order of the signal filter 205.Further, the signal filter 205 need not be implemented as simple lowpass and high pass filters. Again, the application will dictate whichfrequencies require filtering. A signal filter circuit 206 filters theoutput current of the photocells 202 into a signal of interest. Theamplifier 208 amplifies the signal of interest for processing by othercircuitry.

As shown in FIG. 5, where like structure is indicated with likereference, another embodiment of the light detecting system 200 isillustrated. As illustrated, the signal filter 206 of FIG. 4 is replacedwith a first variable impedance load 210 and the signal filter 205 isreplaced with a second variable impedance load 212. The first variableimpedance load 210 provides a low impedance load to the photocells 202when the photocells output a signal of no interest, such as a d.c. orlow frequency signal. The first variable impedance load 210 furtherprovides a high impedance load to the photocells 202 output when thephotocells output a signal of interest, such that the signal of interestwill not appreciably be lost to the first variable impedance load 210.The second variable impedance load 212 provides a high impedance load tosignals of no interest, such as d.c. and low frequency signals, andprovides a low impedance load to signals of interest such that thesignals of interest are amplified by the amplifier 208.

It will be appreciated that the present invention effectively filtersthe effects of the daylight. Also, although shown schematically ashaving a single light detector, in practical applications the circuitsof the present invention are scalable and can accommodate any number oflight detectors or photocells.

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims.

What is claimed is:
 1. A receiver circuit comprising: a light detectorarranged to provide an output signal that varies based upon theintensity of light measured thereby; a constant bias circuit arranged tosupply a reverse voltage across said light detector, said reversevoltage remaining substantially constant irrespective of said outputsignal; a signal filter coupled to said light detector arranged tofilter said output signal; and; an amplifier arranged to amplify saidoutput signal filtered by said signal filter.
 2. A receiver circuitcomprising: a photocell arranged to provide an output signal that variesbased upon the intensity of light measured thereby; a first filtercoupled to said photocell to filter said output signal; a constant biascircuit arranged to supply a reverse voltage across said photocell, saidreverse voltage remaining substantially constant irrespective of saidoutput signal; and an amplifier arranged to amplify said output signalof said light detector.
 3. The receiver circuit according to claim 2,further comprising a plurality of photocells coupled to the constantbias circuit.
 4. The receiver circuit according to claim 2, in whichsaid first filter implements a lowpass filter.
 5. The receiver circuitaccording to claim 4, wherein said first reference voltage is filteredby a power supply noise rejection of said first operational amplifier.6. The receiver circuit according to claim 2, in which said first filterimplements a bandpass filter.
 7. The receiver circuit according to claim2, wherein said constant bias circuit comprises a first operationalamplifier having a non-inverting input coupled to a first referencevoltage, and an inverting input coupled to said photocell.
 8. Thereceiver circuit according to claim 2, in which said first filter isconfigured to filter said output signal to attenuate frequencies outsidea range of frequencies defining a signal of interest and substantiallyallow said signal of interest to couple to said amplifier.
 9. Thereceiver circuit according to claim 2, in which said first filter has afrequency response that notches at an expected frequency range of laserlight intended to be detected by said photocell; and further comprisinga second filter coupled between said photocell and said amplifierconfigured to filter said output signal to attenuate frequencies belowsaid expected frequency range of laser light.